ETHER. 



If the temperature of the atmosphere were everywhere 0* C., and if it were 

 in equilibrium about the earth supposed at rest, its density at an infinite distance 

 from the earth would be 3x10"** which is about T8 x 10 W times less than 

 I <rttirf^ density of the aether. In the regions of interplanetary space the 

 dsositr of the sHher is therefore very great compared with that of the at- 

 taaoaled atmosphere of interplanetary space, but the whole mass of aether 

 a sphere whose radius is that of the most distant planet is very small 

 with that of the planets themselves *. 



Tkt <s*Arr distinct from gross matter. When light travels through the 

 it is manifest that the medium through which the light is propagated 

 not the air itself, for in the first place the air cannot transmit transverse 

 vibrations, and the normal vibrations which the air does transmit travel about 

 million times slower than light. Solid transparent bodies, such as glass 

 and crystals, are no doubt capable of transmitting transverse vibrations, but 

 the velocity of transmission is still hundreds of thousand times less than that 

 with which light is transmitted through these bodies. We are therefore obliged 

 to suppose that the medium through which light is propagated is something 

 distinct from the transparent medium known to us, though it interpenetrates 

 all transparent bodies and probably opaque bodies too. 



The velocity of light, however, is different in different transparent media, 

 and we must therefore suppose that these media take some part in the process, 

 and that their particles are vibrating as well as those of the aether, but the 

 energy of the vibrations of the gross particles must be very much smaller 

 than that of the aether, for otherwise a much larger proportion of the incident 

 light would be reflected when a ray passes from vacuum to glass or from 

 to vacuum than we find to be the case. 



Rrfatirt motion of the tether. We must therefore consider the jether within 

 bodies as somewhat loosely connected with the dense bodies, and we 

 hare next to inquire whether, when these dense bodies are in motion through 

 the great ocean of aether, they carry along with them the {ether they con- 

 tain, or whether the aether passes through them as the water of the sea 

 passes through the meshes of a net when it is towed along by a boat. If 

 were possible to determine the velocity of light by observing the time it 

 takes to travel between one station and another on the earth's surface, we 

 See Sir W. Thomson, Trans. R. S. Edin. Vol. xxi. p. 60. 



